The present invention relates to a headbox for a machine for the manufacture of fiber webs from a pulp slurry, and in particular for the manufacture of paper webs. Such a headbox is described in U.S. Pat. No. 5,034,101 by the inventors herein, and the present invention is an improvement thereover.
One essential element of this headbox is the movable channel wall which defines one side of the pulp discharge channel. The movable wall is stiffened by a box-shaped channel-wall support. In order to counteract the pressure of the slurry prevailing in the outlet channel and the bending of the channel-wall support which results therefrom, a supporting member is provided. Between the channel-wall support and the supporting member, a pressure cushion is arranged to counteract the pressure of the slurry on the movable channel wall.
The above patent describes measures which are intended to avoid the introduction of a moment of flexure into the movable channel wall as a result of the supporting force of the lift device. For this purpose, it suggests the following. The lift device acts directly on the channel wall at each end of the movable channel wall. The line of action of the lift device is so arranged that it extends through the end point of the width of the channel. In other words, the distance between the lines of action of the two lift devices arranged on the operator and drive sides of the headbox is equal to the width of the channel.
However, reducing this proposal to practice is considerably difficult since the pressure cushion must extend substantially over the entire cross machine length of the channel-wall support and of the supporting member. In other words, the length of the pressure cushion must be at least approximately equal to the width of the channel. Furthermore, an easily flexible connection must be provided at each end of the channel-wall support between the channel-wall support and the supporting member. Also, as seen in front view, the central axis of the flexible connection must extend through the end point of the width of the channel. All of these are necessary in order for the channel-wall support to be kept free of flexure and for the inside width of the pulp outlet slot to be maintained constant in the transverse or cross machine direction. However, at each end of the channel-wall support, the lift device cannot extend directly along its line of action when that line extends through the end point of the channel width since, in that case, it would collide at least with the pressure cushion.
The aforementioned publication also describes a manner of construction in which the line of action of the lift device is arranged outside the channel width. In that case, however, the lift device is not coupled to the channel-wall support but instead to the supporting member. In this way, the supporting force of the lift device can be introduced free of bending moments into the movable channel wall. However, this requires a complicated and expensive development of the flexible connection which connects the channel-wall support to the supporting member, since the flexible connection must be adapted to transmit high pressure forces in the event of the failure of pressure in the pressure cushion.